Set initial input for ContainerCheckedTreeViewer

I have trouble setting the initial input for a ContainerCheckedTreeViewer in an Eclipse ViewPart. I set the initial input inside the createPartControl method, but the content is not rendered/visible. This is only an issue when the view is created (e.g. Eclispe startup/opening the view for the first time). Setting the input (e.g. by an oberservable) after the view is fully initialized works and the tree is rendered correctly.

Here is my part createPartControl method which does not work and the update method which works.